Everything that hinders the mastery of nuclear fusion

On August 8, 2021, in California, the National Ignition Facility wrote an important page in the mastery of nuclear fusion. As CEA expert Daniel Vanderhaegen (who is not part of the American team) deciphered in a previous article in Sciences et Avenir, this Californian experiment produced a quantity of energy close to the ignition threshold. That is to say that we are approaching the level where the quantity of energy produced will be greater than that necessary to make the fusion. Exceeding it is now only a matter of months.

But the issue of the ignition threshold is not the only issue to achieve the possibility of industrial development. Because producing electricity with fusion remains "inaccessible with our current technologies, but in a few decades, if a decision to move towards (this technology) is taken, it is possible", explained Mr. Vanderhaegen. Other challenges, indeed, are yet to be overcome. How to manage plasma instabilities, hot molten matter? The transport, manufacture and processing of fuel? And the irradiated materials?... Sciences et Avenir reviews the puzzles that still hinder the advent of what is often called "the energy of the future" .

A mixture of deuterium and tritium is used as fuel for nuclear fusion

The fusion reaction consists of bringing together two light atomic nuclei to create a heavier one: this is the process that powers the core of stars, such as our Sun. Reproducing favorable conditions for fusion requires heating the fuel to a few hundred million degrees Celsius. For this, scientists mainly use two techniques: magnetic confinement and inertial confinement. It is in this last category that the record was broken this summer by the Californians of the National Ignition Facility.

But regardless of the method used, the fuel remains the same: a "DT" mixture of deuterium and tritium, two isotopes of hydrogen. They contain respectively in their atomic nucleus, in addition to the proton, one and two neutrons. Brought to a sufficient temperature, the mixture can begin to fuse: the deuterium and tritium nuclei come together, and produce a helium nucleus and a neutron. The helium is then used to maintain the fusion, by maintaining a sufficiently high temperature thanks to the collisions it makes with the fuel. As for the neutron, it contains 80% of the energy produced by the reaction, i.e. 14 million electron-volts (MeV), but we will come back to this later.

Unlike nuclear fission, only a small amount of fuel is needed: the same amount of energy can be produced with one gram of material in nuclear fusion as with one kilogram of uranium in fission, or ten tons of oil burned in a thermal power plant! The fact remains that the extraction of this deuterium - found in its natural state in the oceans - can only be achieved at the cost of rigorous and precise operations. "These are controlled industrial processes, the deuterium is extracted by electrolysis of heavy water, like when we remove hydrogen from ordinary water, comments André Grosman, deputy director of the Research Institute for Fusion by Magnetic confinement (IRFM, CEA) The industrial production of heavy water, i.e. water in which deuterium replaces hydrogen, has been implemented since the beginnings of nuclear research, because it is used as a neutron retarder in Canadian and Indian 'CANDU' type nuclear reactors".

Tritium must be produced from lithium

Tritium, on the other hand, does not exist naturally. Indeed it has a short lifespan due to its radioactivity, about ten years, hence its spontaneous disintegration. It is produced from lithium when it is bombarded with neutrons: the reaction then generates a tritium atom and a helium atom, which are then separated. Being an alkali metal, lithium reacts strongly with water and air, and is therefore only found in the form of ionic compounds, which are found in abundance in the earth's crust. Also used for batteries in various electronic instruments such as our computers, mobile phones or now in electric cars, it is extracted chemically or from salt deserts, mainly in South America and Australia. A project to build an extraction mine in Germany is currently underway, which would allow the European Union to avoid all the costs and difficulties of delivery.